Class D β-lactamase (also known as oxacillinase) (OXA)

Class D β-lactamases (oxacillinases) are serine hydrolases that confer bacterial antibiotic resistance by breaking down β-lactam antibiotics, including penicillins, cephalosporins, and carbapenems.[1][3] These enzymes belong to a distinct subset of the β-lactamase superfamily and operate through a sophisticated serine-dependent catalytic mechanism involving a carbamylated lysine residue that facilitates hydrolysis of the characteristic β-lactam ring found in these antibiotics.[1][4] Originally recognized for their ability to hydrolyze oxacillin and related compounds, Class D β-lactamases are now a major clinical concern due to the emergence of OXA carbapenemases, which can degrade last-resort carbapenem antibiotics in Acinetobacter baumannii and other gram-negative pathogens.[3][4] These enzymes represent a significant therapeutic challenge because their rapid hydrolysis of antibiotic substrates, combined with secondary resistance mechanisms and the ability to spread via plasmids across bacterial species, makes infections difficult to treat.[2][3] Efforts to develop new antibiotics and β-lactamase inhibitors target Class D enzymes as a key strategy to combat rising antibiotic resistance.

Other names

OxacillinaseOXA enzymeClass D serine β-lactamaseSerine oxacillinase

The enzyme operates through a serine-dependent catalytic mechanism involving four key steps: 1. Nucleophilic activation — Carbamylated lysine (Kcx70) deprotonates serine 67, enabling it to attack the carbonyl group of the β-lactam ring and form a tetrahedral intermediate 2. Intermediate collapse — The tetrahedral intermediate collapses, cleaving the C-N bond through a proton relay cascade 3. Water-mediated hydrolysis — The carbamylated lysine deprotonates water, which attacks the covalently attached intermediate in a nucleophilic addition 4. Enzyme regeneration — The tetrahedral intermediate collapses, eliminating serine 67 and releasing the hydrolyzed product while regenerating the catalytically active enzyme This mechanism results in rapid hydrolysis of the acyl-enzyme intermediate, preventing the enzyme from becoming inhibited by β-lactams in the same way as penicillin-binding proteins.[1]